On Molecular and Microscopic Science, Volume 1 (of 2)

by Mary Somerville

In autumn, the common fly, though quite dead, may be seen adhering to many parts of a room, especially to the window glass, as if it were alive. In this state it is always surrounded by a halo about an inch in diameter of whitish dust, consisting of the spores of the Empusa Muscæ, or fly fungus. The body of the fly is much distended, the rings of its abdomen are separated by the growth of the mycelium from within, and all the contents of the body having been consumed by the parasite, nothing remains but a hollow shell with a thin felt-like layer of the interlaced mycelia of innumerable fungi, for the fly fungus increases with wonderful rapidity within the insect. Mr. Berkeley believes the fly fungus to be merely a condition or phase of one of those anomalous moulds which grow on dead fish, making them conspicuous as they float on the surface of the water, by the foggy halo which surrounds them. Different kinds of parasitic fungi may exist at the same time. Dr. Leidy found a variety in the stomach of the Passalus cornutus, a beetle that lives upon decayed wood. Fungi do not attack the carnivorous beetles.

Man is not exempt from these parasites. Fourteen different species of fungi were discovered by Mr. Hogg in as many cutaneous diseases. There cannot be the smallest doubt of cutaneous disease being induced by inoculation with fungi; merely rubbing certain species on the skin is sufficient. Fungi cause baldness by fixing themselves on the roots of the hair, and destroying the internal structure of the bulb. Our eyes are not exempt from attacks of these parasites, for in performing an operation upon a diseased eye Dr. Hannover found several species of fungi in it; one of them was globular, and strongly refracted the light. There is a fungus consisting of from four to sixty-four cells united in square groups, which infests the stomachs of men and animals, even in a healthy state; but although fungi produce certain cutaneous diseases, there is no proof as yet that fever, cholera, or any other epidemic, is owing to the spores of the fungi which we inhale from the atmosphere.

The family of Coniomycetes consists of six groups, two of which are parasites on living vegetables, the other four growing on those which are dead, decaying, or dying. They are microscopic plants, and their mycelium is filamentous, or vesicular, often obsolete; short threads rising from it bear on their tips either septate spores, or spores like fine dust, inclosed in oval or bottle-shaped cases, called perithecia, or in cells united in a cell, like a necklace of beads. We are chiefly indebted to M. Tulasne and his brother for the obscure and extraordinary life-history of these fungi.

The parasites on living plants form the two vast groups of Epiphytes and Entophytes. The Epiphytes exhibit their fructification on the surface of the plant, while their mycelium penetrates the moist texture of its interior, which feeds them. All parts are liable to be attacked by these fungi; they may insinuate their mycelium into the leaves, stem, flowers, stamens, anthers, and the very heart of the seeds. The mycelium is generally annual, but sometimes it is perennial, and leaves a crop of fungi year after year; it disintegrates the tissues of the plant on which it feeds, and distorts or kills it. Occasionally, the chlorophyll in the leaves is oxidized, and becomes yellow by the oxygen which the parasite absorbs.

The Entophytes, which constitute the second group of parasites on living plants, form microscopic congregations in the interior of the leaves and tender shoots, the only indication of their existence being a white, red, or orange coloured spot, which usually becomes black or brown when the fungus attains maturity. It appears that the same individual of these entophytes may assume two or more different forms during the course of its life, and bear two or more totally dissimilar types of fructification. Of these, the sub-orders Pucciniæ and Uredines furnish many examples.

Fig. 32. Pucciniæi:—a, Aregma speciosum; b, Xenodochus paradoxus; c, Puccinia Amorphæ; d, Triphragmium dubens; e, Young spores of an unknown Puccinia; f, Puccinia lateripes.

[Fig. 32] represents various species of Pucciniæi, which consist of a thread ending in club-shaped or elongated cells called asci, containing a definite or indefinite number of septate conidia or spore dust cells. Each order of plants, as the Rosaceæ, has its own form of these entophytes. In the tissue of a rose leaf, immediately beneath a bright golden coloured surface spot, M. Tulasne found two distinct forms of fungi, living together in a small cavity. The forms were exactly those of a Puccinia and Uredo. The Puccinia consisted of a short colourless stem, ending in a club-shaped cell, containing two conidia or spore cells. These fungi were crowded together in multitudes in a small space so as to form a solid rounded mass, with their broad tops immediately under the skin of the upper-side of the leaf, but sometimes they were arranged in concentric circles. The Uredines, on the contrary, had colourless branching stems, like threads, bearing on their tops pointed spore sacs. In some species these Uredines are scattered through the mass of Puccinia, in others disposed in a circle round it, or in the centre of the concentric ring where the Puccinia takes that form. It was long believed that these two forms of fungi living together in the same cavity were totally different plants, but as in various instances M. Tulasne perceived that the Uredo had sprung up, shed its spores, and vanished before its companion had ripened its fruit, he concluded that the two different forms are merely two states of the same plant, that the larger spores of the Uredo thus early matured, immediately germinate and produce the Puccinia, whose fine dust-like spores are merely the secondary fruit of the Uredo. These minute spores issue through a pore in the conidia or dust cell and a puncture in the upper skin of the leaf into the air, whence they are wafted in myriads by the winds; and, if not too late in the season, they enter into the pores of the leaves and tender parts of the same or other plants that may suit them, and within these they form a mycelium, and produce a young Uredo. Even if the autumnal leaves fall in a moist place before the spores have germinated, the entophyte will grow on the approach of spring, and ultimately send its dust spores to enter into young leaves, and grow with their growth.

Although there cannot be a doubt of the existence of the Uredines as a numerous natural family, M. Tulasne considers the species of certain genera to be only secondary forms of certain genera of Pucciniæ. Many of these minute fungi have a third and even a fourth order of fruit; the principle being carried to a maximum in the order Cæomacei. These entophytes have a delicate mycelium, which gives rise to short or obsolete fertile threads, terminated by single spores or chains of spores. These spores when they germinate produce a second order of spores; these occasionally produce a third order, and so on successively even to a fourth or a fifth order. It is always the last and smallest spores which reproduce the plant. The object of the successive orders seems to be to diminish the size of the spores and to increase their number, that they may more easily enter the stomates of the plants they live upon, and be more easily and widely dispersed by the winds.

The Uredo candida, or Cystopus candidus, which takes its name from the white spot it forms on the leaves of the cabbage and other vegetables, is found to produce both female or germ cells and spermatozoids. Long before the white spot is formed on the leaf, the presence of the abundant spawn may be perceived by swellings and deformities in the victim plant. Its filaments, which creep exclusively in the intercellular canals of the cellular tissue, are tubular, of unequal diameter, and exceedingly branched, and are always formed of cellulose, either thick-walled and gelatinous, or thin-walled and membranous. From this mycelium, little threads hang down, ending in globular vesicles containing a nearly homogeneous colourless matter, and ultimately an aqueous liquid; they are supposed to fix the mycelium to the cells of the victim. According to the examination of Mr. Berkeley, M. Tulasne, and others, the branches of the mycelium accumulate in a hollow immediately under the white spot in the skin of the leaf of the plant attacked. From these branches spring bundles of club-shaped tubes, directed perpendicularly towards the upper skin of the leaf, and forming a tuft or little cushion of variable extent. The summit of each of these club-shaped tubes is formed into a conidium, or spore dust cell, which separates itself from that below it by taking a globular form. In the upper end of the remainder of the tubes, new spore dust cells are formed, and so on indefinitely. These conidia remain attached to one another in a string by slender constrictions which become thinner, and at last give way from above downwards, and they escape in succession through a crack in the skin of the leaf. The quantity of spores that are generated by the dense mass of these club-shaped tubes must be enormous.